Varying inner diameter collector electrode for an electron beam tube,particularly high powered travelling-wave tubes



Jan. 7, 1969 RHWOLFRAM 3,421,036

VARYING INNER DIAMETER COLLECTOR ELECTRODE FOR AN ELECTRON BEAM TUBE, PARTICULARLY HIGH POWERED TRAVELLING-WAVE TUBES Filed Sept. 21, 1966 Sheet of 2 l l I 1 INVENTOR ROLAND WOLFRAM BY diedmonnsvs R. WOLFRAM 3,421,036

HIGH POWERED TRAVELLING-WAVE TUBES Jan. 7, 1969 M f O E t e e h S VARYING INNER DIAMETER COLLECTOR ELECTRODE FOR AN EL TUBE, PARTICULARLY Filed Sept. 21. 1966 w T N E V m ROLAND WOLFRAM ionuzvs.

United States Patent Germany Filed Sept. 21, 1966, Ser. No. 581,097 Claims priority, application Germany, Sept. 21, 1965,

s 99,535 US. Cl. 313 s9 4 Claims Int. Cl. H013 29/10 The invention relates to an electron beam tube, in particular to a travelling-wave tube of high power, with a cup-shaped collector electrode which receives the electron beam, such collector electrode being arranged coaxially with respect to the beam axis and presents a rotationally symmetrical inner space which has a varying internal diameter and in which the electron beam diverges concentrically to the beam axis.

It is a known fact that velocity modulated tubes, for example, such as klystron amplifier tubes or travellingwave tubes, require a collector electrode for the electron beam. Generally such collector electrodes are in the form of a cup-shaped metal part which is arranged coaxially with respect to the electron beam axis and consists of a material with good heat-conducting capacity, for example of copper. The electron beam tends to generally uniformly diverge within the cup-shaped metal part so that the majority of the electrons strike upon the lateral wall-plane of the collector electrode.

With regard to known collector electrodes of the described type, the heat occurring through the impact of the beam electrons upon the collector wall, may not exceed a certain value if impermissible high temperature peaks are to be avoided. The local heating of the collector is dependent on the power-density of the electrons applied to the inner wall of the collector electrode. If the rotationally symmetrical collector has, corresponding to the customary constructions, an inner wall extending parallel to the tube axis, the power density is the greatest, assuming that the electron beam continues to spread out, on the collector wall at the point of impact of the electron beam edge or periphery and then decreases towards the collector bottom, while it can again assume larger values on the actual bottom of the collector. Consequently, the power density of the electron bundle is, in this case, distributed irregularly over the length of the collector electrode. As a result, in order to avoid local over-heating the collector must be larger than otherwise required for the reception of the total power of the electron beam.

Electron beam tubes with rotationally symmetrical collector electrodes are also known, in which the inner diameter thereof becomes steadily or gradually smaller toward the electron entrance. It is possible that such a collector electrode is loaded more uniformly as to the beam power towards the collector bottom than a collector with a cylindrical inner space, assuming a steadily diverging electron beam. However, an uneven distribution of the power density still remains which may "become impermissibly high, especially in the front portion of the collector electrode.

Furthermore it is a known feature to taper the inner space of the collector electrode of a travelling-wave tube in a direction towards the delay-line in a wedge-shaped manner in order to make more difficult the discharge of secondary or reflected electrons out of the collector electrode. In this case, a smaller loading of the collector may occur close to the electron beam edge, if it strikes the conical part, than in the case of a collector electrode with ice axis-parallel inner wall. However, in no event is a uniform power density achieved of the electrons arriving at the collector wall in the back part of the collector electrode.

It is the purpose of the invention to produce a collector electrode for electron beam tubes in which the power contained in the electron beam to be received is substantially uniformly distributed over the collector wall. In order to solve this problem, it is proposed in an electron beam tube of the type initially mentioned, according to the invention, that the contour lines of the inner space of the collector electrode follow approximately the differential equation =%=r. a (aw d) wherein N is the direct-current power contained in the electron beam during its entrance into the collector electrode, n is the permissible power density during electron bombardment on the inner wall of the collector electrode, z is a coordinate in longitudinal direction of the collector electrode, r designates the respective inner radius of the collector electrode, a designates the respective angle between the inner wall of the collector electrode and the beam axis, r as function of z represents the respective beam radius, considered as if the edge of the electron beam would spread out without hindrance after impact upon the collector electrode, and (,o designates the appropriate angle between beam edge and beam axis.

In the determining equation for the contour in longitudinal-section of the inner wall of the collector, proposed according to the invention, N as the direct-current power of the electron beam upon its entrance into the collector, and m as the maximum permissible power density on the collector wall, are respectively constant values which result from the structure and the operating characteristics of a particular tube. The functions r (z) and %(Z) which characterize the electron beam course may be determined for each tube installation by experiments or by calculations. The desired contour line for the inner space of a collector electrode, the inner diameter of which changes in dependence on the z-axis, may be determined by a graphical approximation solution or by means of an analog computer. The configuration of the collector electrode obtained in this manner assures that the power contained in the electron beam is distributed practically completely uniformly over the inner wall of the collector. This eliminates local power peaks on the collector wall during electron reception so that the collector electrode may be of smaller construction than the known pot-shaped elec tron beam collectors.

When constructing the collector electrode of an electron beam tube according to the invention, it is preferable to proceed from the feature that in the center range of the inner collector space at the location of a small, axisparallel wall zone ((1:0), a maximum inner radius exists. The selection of this maximum inner radius depends upon the structure of the collector electrode and upon the total power contained in the electron beam, the radius fulfilling the function ao gab zvo (20) In this equation, Z0 designates the abscissa at which the electron beam edge strikes the wall surface with oc=0.

Expediently, the respective inner radius of the collector electrode is selected somewhat larger than the calculated beam radius ahead of the point at which the beam edge impacts. The advantage of this measure consists of the feature that a sufficiently open space exists for possible deviations of the beam course from its supposed value which may be, for example, based upon variations when the electrons are shot in.

The invention will be explained in greater detail in the following, in connection with the drawing, in which:

FIGURE 1 illustrates the principle of construction of the collector electrode of a velocity modulated tube in accordance with the invention;

FIGURE 2 is a chart illustrating operating characteristics of a tube employing the invention; and

FIGURE 3 is a schematic figure, similar to FIGURE 1, illustrating a collector having an internal contour in accordance with the invention.

Referring to FIGURE 1, the reference numeral 1 designates a cup-shaped collector electrode which has an inner contour with a rotationally symmetrical configuration of varying diameter. An electron beam 2 which endeavors to continuously spread out within the collector electrode according to the beam edge curve 3, enters such collector electrode which may, for example, consist of copper and is to be constructed as a part of the discharge vessel of an electron beam tube. Assuming that the electron beam 2 is not further braked within the collector electrode 1, that the power density in the beam cross section is essentially constant, and that the individual electron paths resemble each other, the following formula is applicable for the power density of the collector electrode:

As an approximation, this formula may be simplified as follows:

7 M2) z h.(fa.

This approximation is sufficiently accurate that, up to -003%? with p 15, the error remains than 2%. In the equation N is the direct-current power of the electron beam in the collector electrode, while all other values are functions of the coordinate z in longitudinal direction of the collector electrode, and that is to say, n(z) represents the power density on the collector wall; r (z) represents the calculated value of the beam radius following impact of the beam edge upon the collector wall; (z) represents the angle which is enclosed by the beam edge with respect to the beam axis; r represents the inner radius of the collector and a the angle which is enclosed by the inner wall of the collector with respect to the abscissa z. From this approximation for the power density n(z) it may be derived that a uniform power density n on the inner wall of the collector electrode is obtained when the contour lines of the inner space of the collector electrode, according to the invention, follow the differential equation This differential equation may be easily solved by the feature that every electron collector, irrespective of the particular configuration of the inner space, must have a zone, however small, at which the collector wall, in cross section, is parallel to the axis, and at this point, with appropriately selected radius, the permissible power density is not exceeded. The abscissa Z in FIGURE 1, at which this may be the case, follows from the specified differential equation with dc=0 as:

In this equation, r designates a maximum collector radius, the selection of which depends upon the power N and upon the entire construction (collector material, cooling, etc.). The near or inner part of the collector may be divided into equal sections Z0, Z1, 2 etc. Proceeding from 2 a starting tangent with the angle e 0 is drawn to Z1.

At Z1 the value a may then be computed according to the formula min This principle may be continued according to the known tangent lay-out method. \Vith regard to the ascertainment of the collector contour ahead of the point Z it should be observed that generally the inner radius of the collector electrode 1 will not coincide with the beam radius r at the abscissa z but aready lies within the beam-edge. Consequently, with regard to values z z there would be the danger that the permissible power density is exceeded at the beam edge. Consequently when the differential equation proposed according to the invention is solved graphically, it is recommended to begin with a chord extending towards the collector mouth and to then continue to calculate according to the chord lay-out method. However, in most cases it is sufiicient to compute the wall slope at, with r r directly at the beam edge according to the equation as a function of z. The individual calculated values for the Wall slope or, along the beam edge produces a zone out of the directional field of the wall. The contour expediently is so laid out that on the one hand it corresponds with the directional field and on the other hand it leaves a sufficiently open space with regard to the theoretical value to enable accommodation of possible variations during the shooting-in of the beam.

A practical constructional example for the collector electrode of a travelling-wave tube according to the invention is hereinafter described. This example is based upon a direct-current power loss N of 4.1 kw., while the construction of the tube provides a maximum inner radius r :l.7 cm. On the basis of a known magnetic field course within the collector electrode, which is partially shielded magnetically, in known manner, by a shielding cylinder of soft-magnetic material, the beam course r (z) and p (Z) was computed by means of an analog computer in the manner illustrated in FIGURE 2. In this figure, the path distance 2. is presented on the abscissa, measured in cm., i.e., measured from the cathode surface of an appropriate travelling-wave tube, while the ordinate presents the beam radius r in cm. and the angle of the beam radius (p degrees. FIGURE 2 additionally illustrates the course of 11, (p /l and /r above the abscissa 1. With a permissible power density of n =100 W/cm. and the given values for r and N the expression z: 75, 76, 77, 78, 79, cm. r t 1.7, 1.65, 1.45,1.l5, 0.83, 0.45 cm. a: 3.5, l0, l4.4, l6.8, l8.9, -22.1 degrees There is obtained thereby a tangent drawing which may be simplified with respect to the practical construction. In the front part of the collector, individual values for a, along the beam edge r are respectively computed with the following results:

z=70, 71, 72, 73, 74 cm. a :8.5, 7.4, 5.7, 6.5, 4.8 degrees The directional tangents 5 corresponding to the values for a, are drawn in FIGURE 3 along the beam edge course which corresponds to the curve r of FIGURE 2 and which is, in this case, represented by the dot-dash curve 4. A collector contour, corresponding to the dashline 6 was then selected proceeding from Z in the range z z said contour to correspond with the slope of the directional tangents 5, as well as to leave free an open space for variations in the beam course. Following the value z the tangent drawing determined above is represented as a continuous line 7. In comparison therewith, the collector contour, as actually constructed, which is also represented for the values z z as dash-line 6, is somewhat simplified. In this case, the circular enlargement at the collector bottom accommodates itself to the fact that an ideal peak is not possible and that the heat occurring at the collector bottom because of this is advantageously uniformly conducted off towards several sides. In addition to this, in FIGURE 3 the reference numeral 8 designates a soft-magnetic shielding cover which is provided to magnetically shield the inner space of the collector with respect to a magnetic guidance-field which is customarily required, at least partially, for the bundled guidance of the electron beam of a travelling-wave tube so that a spreading out of the beam within the collector electrode on the strength of the space-charging forces is possible.

Changes may be made Within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

I claim:

1. An electron beam tube, such as a travelling-wave tube of high power, with a cup-shaped collector electrode for receiving the electron beam, which collector electrode is arranged coaxially with the beam axis and pos sesses a rotationally symmetrical inner space which has a varying inner diameter, and in which the electron beam steadily diverges concentrically to the beam axis, characterized in that the contour lines of the inner space of the collector electrode follow approximately the dilferential equation in which equation N is the direct-current power contained in the electron beam at its entrance into the collector electrode, n is the permissible power density during electron impact on the inner wall of the collector electrode, 1 is a coordinate in longitudinal direction of the collector electrode, r represents the respective inner radius of the collector electrode, on represents the respective angle between the inner wall of the collector electrode and the beam axis, r represents, as function of z, the respective beam radius, considered as if the edge of the electron beam would diverge, without hindrance after impacting the collector electrode, and represents the appropriate angle between the beam edge and beam axis.

2. An electron beam tube according to claim 1, wherein the collector electrode possesses, in its center range, at one point 1 a maximum inner radius r which, with oc=0 fulfills the relation 3. An electron beam tube according to claim 2, wherein the inner radius of the collector electrode, ahead of the point Z is selected in such a manner that an open space remains between the supposed value of the beam edge r (z) and the inner wall of the collector electrode.

4. An electron beam tube according to claim 1, wherein means is provided for magnetically shielding the collector electrode.

References Cited UNITED STATES PATENTS 2,894,169 7/1959 Kreuchen 315-538 HERMAN K. SAALBACH, Primary Examiner.

PAUL L. GENSLER, Assistant Examiner.

U.S. Cl. X.R. 

1. AN ELECTRON BEAM TUBE, SUCH AS A TRAVELLING-WAVE TUBE OF HIGH POWER, WITH A CUP-SHAPED COLLECTOR ELECTRODE FOR RECEIVING THE ELECTRON BEAM, WHICH COLLECTOR ELECTRODE IS ARRANGED COAXIALLY WITH THE BEAM AXIS AND POSSESSES A ROTATIONALLY SYMMETRICAL INNER SPACE WHICH HAS A VARYING INNER DIAMETER, AND IN WHICH THE ELECTRON BEAM STEADILY DIVERGES CONCENTRICALLY TO THE BEAM AXIS, CHRACTERIZED IN THAT THE CONTOUR LINES OF THE INNER SPACE OF THE COLLECTOR ELECTRODE FOLLOW APPROXIMATELY THE DIFFERENTIAL EQUATION 